OK, so I was looking for Rit dye, but couldn't find it anywhere (I'm in Canada), so my wife found some Tintex.
Bought a couple of "black" packages (using quotes on purpose there), and uh, well, the results are more of a deep purple.
Actually not a bad color, if you're aiming for purple, but oddly enough, when I bough the "black" packages, I was expecting ... black.
It wasn't a concentration thing, since I used 2 packs, in a container barely large enough to hold the chassis - plenty of dye. (My wife hasn't seen the results on the container yet, but that's a different fight.)
Anyway, here's a shot - maybe I'll leave it. In some lighting it actually looks black, but in sunlight it is definitely purple. May be tough to see in the photo, but the couch is black, and the dye package is (supposed to be) black.
I'll see if I can find some Rit dye - have to decide if I'll keep it as-is or not. I guess take this as a warning re Tintex.